Implantable infusion apparatus of the general type with which we are concerned has been in use for a number of years to treat diabetes, Alzheimer's disease, certain types of cancer and other chronic diseases. Basically, the apparatus includes a housing which contains a collapsible infusate reservoir. An inlet port through a wall of the housing communicates with the interior of the reservoir and that passage is closed by a needle-penetrable septum mounted in the housing wall. An outlet passage from the reservoir containing a flow restrictor extends to the housing exterior where it is connected to the proximal end of a flexible catheter.
In use, the apparatus is implanted at a selected location in the body so that the inlet septum is situated directly underneath the patient's skin and the distal end of the catheter is positioned at a selected infusion site. Infusate is delivered to the infusion site by forcing that fluid from the apparatus reservoir through the catheter to the infusion site. The flow restrictor in the reservoir outlet determines the flow rate from the reservoir. When the infusate reservoir becomes empty, it may be refilled by injecting fresh infusate through the apparatus' inlet septum. As noted previously, the inlet is accessible readily by transcutaneous injection using a hypodermic needle or cannula.
In the infusion apparatus of interest here, infusate is expelled from the reservoir to the infusion site by collapsing the reservoir. This collapsing force is provided by a two-phase fluid which is situated in a fluid-tight space outside the reservoir. The fluid is both a liquid and a vapor at physiological temperatures, e.g. 98.6.degree. F., and it exerts a positive and constant pressure over the full volume change of the reservoir. On the other hand, when the infusate reservoir is expanded upon being refilled with fresh infusate during the refilling process described above, the two-phase fluid is compressed with a portion of it reverting to its liquid phase thereby recharging that vapor pressure power source. The construction and operation of inplantable infusate apparatus and pumps of this general type are described in detail, for example, in U.S. Pat. Nos. 3,731,681 and 3,951,147 and in the article entitled "Liquid Dispensers" by B. M. Wright in the Quarterly Bulletin and Review, Vol. 16, No. 3, Sept. 1, 1964 and in the Journal of Physiology, Vol. 177, (1965). See also the September 1964 Masters Thesis of P. D. W. Soden to be found at Victoria University of Manchester, England.
While the prior art pumps operate satisfactorily, they are relatively expensive to manufacture and to assemble. Also, they are relatively large. For example, one such pump of which we are aware is in the order of 3.3 inches in diameter, one inch thick and weighs about 220 grams. When that prior prosthesis is implanted in a patient's body, the patient is obviously well aware of its presence and may, as a result, suffer considerable discomfort and anxiety.
Some known implantable pumps are difficult to refill in that it is difficult to locate their septa in order to insert needles into their inlet ports to refill or otherwise service the apparatus. This may be due to a combination of factors, including the use of an inlet septum having a small surface area and the inability to distinguish the septum from the remainder of the implanted apparatus. Even if the spot on the patient's skin directly above the septum is marked by a tattoo when the pump is implanted initially, over the course of time, the relative positions of the mark and the underlying septum may change due to patient movements and weight changes. In those known pumps whose catheters exit the housing close to the septum, a mispositioned needle can actually damage the pump by puncturing the rubber catheter. Such damage would, of course, necessitate surgical removal of the pump.
In this connection, we should mention that when refilling an implanted pump, the normal procedure is to insert a hollow needle into the pump's inlet port and allow any remaining volume of the original infusate in the reservoir served by that inlet port to back-flow out through the needle. Then, a fixed volume of the fresh infusate is injected into the reservoir through the needle, after which the needle is withdrawn. It is apparent, therefore, that each emptying and refilling procedure is a time-consuming process that involves skin penetrations and requires the patient to remain still while the needle fixed to his body introduces and/or removes fluid from the infusion device implanted in his body. In many instances, this procedure is performed in a clinic or physician's office or on a hospital out-patient basis. Therefore, each office visit for servicing the pump can be quite expensive.
Also, some implantable apparatus such as those described in U.S. Pat. Nos. 4,193,397 and 4,258,711 have two-pumping chambers or reservoirs enabling them to dispense two different infusate concentrations or infusates. The two pumping chambers are purged and refilled independently by way of separate inlet ports positioned at different locations on the pump housing, each port having its own needle-penetrable septum located underneath the patient's skin.
Another known implantable infusate dispenser disclosed in U.S. Pat. No. 4,496,343 for example, has, instead of a second pumping chamber, an injection portal on the housing wall. This portal is basically a small chamber with an outlet leading to the catheter and an inlet port closed by a self-sealing septum located underneath the patient's skin. Infusate injected transcutaneously into the portal flows directly to the catheter and, therefore, to the infusion site. In other words, the injection process provides the pumping force to deliver the infusate. Such a device can also be used for blood withdrawal.
It is apparent that the proper servicing and utilization of such dual port devices may require many more skin penetrations than are needed to service a pump with a single inlet port. As noted above, once the device is implanted, the positions of the inlet ports and their septa are more or less fixed with respect to the overlying skin area of the patient. Therefore, over the period of implantation, the patient's skin may be punctured many times at the two septa locations resulting in inconvenience and pain for the patient.
Another disadvantage of the prior plural-port implantable pumps is their propensity for being refilled with the wrong fluid. More particularly, after the device is implanted, as noted above, its position may change somewhat relative to a fixed spot on the patient's skin surface. Also, the septa are quite small and indistinguishable. Therefore, when refilling or purging the implanted device, it is quite easy for a nurse to insert a needle into the wrong inlet port if she is not very careful. In the case of a two chamber insulin pump, for example, this could result in the basal reservoir of the pump being refilled with bolus infusate and the bolus reservoir being charged with lower concentration basal infusate, or it could result in one reservoir of that pump being emptied or filled twice and the other reservoir not being serviced at all.
It would be desirable, therefore, if the number and duration of the transcutaneous injections required to access or to service an implanted pump could be minimized, along with the potential for servicing errors. This would not only reduce the risk of infection to the patient, it would also reduce the incidence of epidermal problems associated with implanted refillable infusate pumps of this type, and it would certainly reduce the physical and emotional stress on a patient required to wear such an implanted device.